EKF with ZED

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I want to fuse the ZED pose with an external GPS. For a start, I am just trying to pass the ZED signal through an EKF (robot_localization) node (I expect to see the same behavior as without the EKF). Therefore, I am disabling "publish_map_tf" (map -> odom), and run an EKF node that subscribes to the pose topic. Although I expect to see no change in behavior, eventually my base_link drifts, as if it has velocity even when I stay in the same place.

my ZED2 parameters are:

# params/common.yaml
# Common parameters to Stereolabs ZED and ZED mini cameras
---

# Dynamic parameters cannot have a namespace
brightness:                 4                                   # Dynamic
contrast:                   4                                   # Dynamic
hue:                        0                                   # Dynamic
saturation:                 4                                   # Dynamic
sharpness:                  4                                   # Dynamic
gamma:                      8                                   # Dynamic - Requires SDK >=v3.1
auto_exposure_gain:         true                                # Dynamic
gain:                       100                                 # Dynamic - works only if `auto_exposure_gain` is false
exposure:                   100                                 # Dynamic - works only if `auto_exposure_gain` is false
auto_whitebalance:          true                                # Dynamic
whitebalance_temperature:   42                                  # Dynamic - works only if `auto_whitebalance` is false
depth_confidence:           50                                  # Dynamic
depth_texture_conf:         100                                 # Dynamic
pub_frame_rate:             15.0                                # Dynamic - frequency of publishing of video and depth data
point_cloud_freq:           30.0 #15                               # Dynamic - frequency of the pointcloud publishing (equal or less to `grab_frame_rate` value)

general:
    camera_name:                zed                             # A name for the camera (can be different from camera model and node name and can be overwritten by the launch file)
    zed_id:                     0
    serial_number:              0
    resolution:                 3                               # '0': HD2K, '1': HD1080, '2': HD720, '3': VGA
    grab_frame_rate:            30                              # Frequency of frame grabbing for internal SDK operations
    gpu_id:                     -1
    base_frame:                 'base_link'                     # must be equal to the frame_id used in the URDF file
    verbose:                    false                           # Enable info message by the ZED SDK
    svo_compression:            2                               # `0`: LOSSLESS, `1`: AVCHD, `2`: HEVC
    self_calib:                 true                            # enable/disable self calibration at starting
    camera_flip:                false

video:
    img_downsample_factor:      1.0                             # Resample factor for images [0.01,1.0] The SDK works with native image sizes, but publishes rescaled image.
    extrinsic_in_camera_frame:  true                            # if `false` extrinsic parameter in `camera_info` will use ROS native frame (X FORWARD, Z UP) instead of the camera frame (Z FORWARD, Y DOWN) [`true` use old behavior as for version < v3.1]

depth:
    quality:                    1                               # '0': NONE, '1': PERFORMANCE, '2': QUALITY, '3': ULTRA
    sensing_mode:               0                               # '0': STANDARD, '1': FILL (not use FILL for robotic applications)
    depth_stabilization:        0                               # `0`: disabled, `1`: enabled
    openni_depth_mode:          false                           # 'false': 32bit float meters, 'true': 16bit uchar millimeters
    depth_downsample_factor:    1.0                             # Resample factor for depth data matrices [0.01,1.0] The SDK works with native data sizes, but publishes rescaled matrices (depth map, point cloud, ...)

pos_tracking:
    pos_tracking_enabled:       true                            # True to enable positional tracking from start
    publish_tf:                 true                            # publish `odom -> base_link` TF
    publish_map_tf:             false                            # publish `map -> odom` TF
    map_frame:                  'map'
    odometry_frame:             'odom'
    area_memory_db_path:        ''
    area_memory:                true                            # Enable to detect loop closure
    floor_alignment:            false                           # Enable to automatically calculate camera/floor offset
    initial_base_pose:          [0.0,0.0,0.0, 0.0,0.0,0.0]      # Initial position of the `base_frame` -> [X, Y, Z, R, P, Y]
    init_odom_with_first_valid_pose: true                       # Enable to initialize the odometry with the first valid pose
    path_pub_rate:              2.0                             # Camera trajectory ...