It happened last weekend at the William Penn Inn in Lower Gwynedd, Montgomery County.
Police have released surveillance pictures of the suspected thief. And based on his clothing, it sure doesn't look like he was a guest at the party.
Here's what happened.
"We were moving all the gifts from gift table into the bride and groom's car," said Jillian Bowie, the groom's sister.
The reception was over. By all accounts, it had been a lovely time at the historic William Penn Inn. Bowie was one of two people moving the gifts to the groom's car.
Among the gifts was a small box shaped like a wedding cake. Bowie calls it a birdcage.
"I brought the birdcage out that contained the checks and the cash and put it in the car - put it in the back seat - and closed the door, walked back into the venue to get more gifts. Within a minute ... it was stolen," she said.
At first she couldn't believe it.
"I really second-guessed myself as to whether... did I not just put this in here?" said Bowie.
But she had.
Lower Gwynedd Police started pouring over surveillance video. It turns out the suspect shows up for 3 hours, says Police Chief Paul Kenny.
"At one point he actually walked into the reception room, looked around a little bit, walked back outside. Just hanging out," said Kenny, adding the man is a friend of neither the bride nor the groom. "Nobody knows him," said Kenny.
By process of elimination, that man is a person of interest. It's believed he may have departed in a gray Mazda 3, also captured by surveillance cameras.
"Young white male with a scruffy beard. He was wearing a green shirt, cargo-type shorts," said Kenny.
As for how much was taken, "I'm going to guess between $3,000 and $5,000," Bowie said.
And the lesson learned?
"Lock your car. I mean it's unfortunate, but crime happens in an instant," said Kenny.