G.B.
In all truthfulness I believe she was scared of your reaction when you found out she urinated in her clothing and was trying to hide the fact by putting them under the chair. Smiling or giggling when being confronted from an angry person is most always a sign of nervousness.
Accidents like this are not uncommon. Every child should be allowed to have a few. My three kids have each had at least a few when they were younger. Usually they happened because the child was engrossed in playing with freinds or something else, so much so that they ignored the urge to urinate. It goes away, but then when the urge comes back, there may be no time to respond and they pee their pants. Or they may laugh which forces the pee out of the full bladder and once the flow starts it cannot be stopped. Heck, even as a teen I would hold my urine and had an accident from laughing once. Children are simple, and many don't have a problem ignoring body signals like hunger, cold, uriating signals, etc. She needs to be encouraged to respond to her body triggers and NOT wait- The body will eventually reabsorb some of that urine which is not good.
The other thing you MUST consider is that she might have a urinary tract infection. Earliest signs would be an OFF smell to the urine, and potty accidents.
Many children may pee their pants, and remove the clothes,without any thought at all that it was a big issue. After all, they think, i took care of the problem. I changed my clothes. You need to look at it from an immature point of view.
I have a son who peed the bed on occassion between 3 and 8 . He would get up, change, and go back to bed. He would litterally forget that he had done that in the middle of the night. I would find his clothes and he would say, "oh yeah, I forgot about those." I found that milk products encourage night time peeing.
Always give the 'young' child the benefit of the doubt. If she is 10 and doing this then you have a whole nother issue on your hands.