Jennifer Lopez -- On the Down Low Again with Casper Smart

Jennifer Lopez and Casper Smart very quietly hooked up Thursday night, leading to the obvious question ...

Here's how it went down. Casper was out at the Z Zegna & GQ event in Hollywood where he hung out with Nick Jonas.

There are plenty of pics of Casper leaving the event with no sign of JLo. But we're told she was actually waiting in the car for him as he left.

They went to Hotel Cafe in Hollywood where this pic was snapped. It's the only pic of the 2 of them that was taken the entire evening.

They supposedly broke up in April, but they've been seen together from time to time, and they've taken great pains to avoid pics. Remember when she ducked down in her car?

And you may recall Casper's dog was at JLo's house recently when the dog bit someone.