No, you aren't imagining it:
As a humble passenger with PPL, it appears to be that the pre-takeoff checks (taxiing onto the runway) include an item to turn on the emergency exit lights. Shortly after takeoff, just after raising the u/c, turn them off again.
but the reason it occurs is related to the position of the "No Smoking" switch:
NO SMOKING switch
ON : The EXIT and NO SMOKING signs come on, with a low tone chime.
AUTO : The EXIT signs come on, when the landing gear is extended, and they go off when the landing gear is retracted. A low tone chime sounds (depending on the CIDS/CAM programming), when the lights go on or off.
The NO SMOKING signs are on.
OFF : The EXIT and NO SMOKING signs are off. A low tone chime sounds, when the lights go off (depending on the CIDS/CAM programming).
YS