Sorry, Podio doesn't store logs for hooks :(
Hook might be un-validated if your service was too slow or down. Podio expects that hook handler is async and simply acknowledge receiving the hook notification. Here is quote from https://developers.podio.com/doc/hooks.
The hook must respond with a 2xx status code. If the status code is
different from 2xx more than 50 consecutive times the hook will return
to being unverified and will have to be verified again to be active.
Additionally, your hook may return to unverified if you do not send
responses in a timely manner. You should handle any heavy processing
asynchronously.
Is there a chance that your service was down or slow recently, may be related to Amazon S3 service disruption on Tuesday (https://aws.amazon.com/message/41926/)?