Webcams and recording?

We have a bit of a thief problem. seems that our till count is short around $20 at times, and though its not much, we can't risk having even more gone. we have installed a webcam on the computer facing the front of the store, and it records just fine, however it only records continuously, while we need it to be be motion detected. having it on like that at night would be a huge save no gigabytes and regular deletes, so if there is any software out there that you know of that would detect motion and begin a screen recording, I'd be really happy to know about it. thank you