There's actually a quirk in the data that suggests that more people do test positive the day after a booster.
I've seen various suggestions from, "people tend not to notice symptoms" on the day or so prior to booster" because of the expectation/stress associated with getting it. To the more direct one that you feel a bit wobbly the following day due to the vaccine kicking in, and that may compound any nascent symptoms so you do get tested. Dunno' really, but you can see the down blip in the data.