We had the same problem, but this seems to have passed now and it didn't last too long. It drove me potty (
all the same though! (Sorry!)
I made a point of making sure that Charlotte sat on the toilet or potty just before going to bed and I tried to encourage her to do both a wee and a poop. Then i'd say to her, look now we've been to the toilet and done our wee-wees and poops, we don't need to get up again do we? This would sometimes work, sometimes not, but like I say it didn't last too long.
I know what you mean about ignoring their requests and then finding that they actually did need to go, I used to worry about that too, but they are all delaying tactics. If you know that they've been to the toilet shortly before going to bed (or just before) then they should be fine.
Good luck! Hope that it doesn't last too long.