Check what kind of weapon they were holding and how far away I was when I saw it, shrug like it was nothing and walk away quickly but calmly with hands in pockets around the corner, then bolt around several other corners to lose them. If it was closer, I'd admit that "I saw nothing, carry on" and "this never took place," and walk away hoping they'd understand.
WWYD if you were given a computer that someone didn't want, didn't know how to program it to your needs and gave it to your computer wizard friend, then that same someone needed the hard drive containing important stuff on it, but your friend already wiped the memory clean to use himself?