Come/crawl out of the woodwork
Definition 1 of 2
Pro Tip 1/3Surprise Appearance
The phrase often implies surprise at seeing someone after a long time, especially if unexpected. 
After his article went viral, old classmates came out of the woodwork to contact him.
Pro Tip 2/3Hidden Motives
People may seem to come out of nowhere, often because they want something from the situation. 
Volunteers came out of the woodwork once the charity offered concert tickets as a reward.
Pro Tip 3/3Opportunistic Context
The phrase is used when people are drawn by an opportunity, not just casually appearing. 
When the startup began getting press, investors came out of the woodwork.