Should i tell my mom when i know my dad isn't where he says he is?

Dad is almost 80... i work for an airline so he flies for free... over the past year he has flown several times to locations just for the day... Reno.. .LA... etc..

he tells my mom that he is driving to a neighboring town with some friends from the health club... or that he is going to a car show or some such.. and then he sneaks off to do God knows what... my mind is working overtime on WHAT he could be up to...

he is in excellent health... sees a dr. regularly... so i don't think it is dementia or anything... Should i tell my mother? i don't want to upset her.. but on the other hand why the duplicity...

he knows that I know as i happen to be working on the flight he took home today, he said OH.. you mom thinks i went to XYZ city... and i said.. well why didn't you? and he said i went with a friend to Reno for the day... there were so many people around that it really wasn't the place for me to question further and when he got off the plane he just said thanks and bye like nothing was wrong.. so maybe i am overreacting... AM I?

i hate drama.. but i hate being in the middle of this too.

thanks for any input... this is an awesome forum!