Two words, Panic button... (The red one OP)

That person saw an opportunity, and took it.

Two lessons. 1. Lock button not unlock. 2. Never keep stuff visible in your car. Either keep no valuables in your car, or if it's unavoidable (like you've been shopping and haven't gone home yet), keep it in the trunk. However, visibly putting things in your trunk before leaving your car unattended can also call the attention of thieves. Not trying to victim blame, just giving suggestions for the future.

Next time press the alarm button or the lock button to find your car, never the unlock button.

That's why you press lock, or press it 2 times to get the horn to honk

Seems like the lock button would've been a better option, given you'd not only see the flash but hear your car too.

That is why you use the lock button when you can't find it.

that's why you hit the lock button. it still lights up but no one can get in. logic man use it.

always push the lock button. it makes the same noise.

I would've chased them down, screaming thief at the top of my lungs, but if you were unable to do that, I understand.