I don't know how old your parents are but I know from my own experience with my own parents and myself as I age we get so forgetful. Sadly my parents did that to me when they were in their 80's giving me stuff then saying I took it. It is senility at it's best and the fact our memories don't work all that well. I've found myself doing it to my kids and I'm in my mid to late 50's. So try to be understanding they may be just forgetting and honestly don't remember. If they are suspicious by nature it will only get worse as time goes on and they age.
Ruth