This has just started in the last couple of days but after she's eaten about half of her food on her tray she'll start taking handfuls and placing it under her highchair tray in her lap. I find it mildly amusing until I go to take her out and she's covered in salmon or broccoli or whatever. I've tried to get her to stop by saying no and stopping her hand when she's in the process of doing it but she doesn't get it, obviously, because she's so young. Do other kids do this? I know dropping food and stuff is pretty normal at this age and she doesn't do that, but I think it's weird that she's "hiding" it in her lap.

Oh, and I thought maybe it meant that she was done eating but about every other handful will go into her mouth. So, whatev.
Re: DD is "hiding" her food. WTF?
Not necessarily "hiding" but over the last couple of weeks DS has gotten really into putting things inside of other things. At meals, he thinks it is so fun to put his food in the little indented area on the tray where the cup should go - sometimes he takes it out of there and eats it, sometimes he doesn't.
Maybe you can put some type of container (they make bowls with suction bottoms) on the tray for her to put stuff in, instead of her lap?
H did this for a while. I don't know why she was doing it, but it definitely wasn't because she was full because like your LO she'd still keep eating.... It seems to have stopped 99% of the time. Now the cool thing to do is dangle something over the edge of the tray for the dog to take from her hand.
We've just started saying "we keep food on the tray. if you don't want it hand it to mommy/daddy or put it right here". That seems to have worked for the most part.