Yes! The vet is waiting...
... thanks for all your replies! This morning she dug a new tunnel, behind a garden shed, where some ivy could hide her mischiefs! I was inside for half an hour, decided to check on her, and bingo! She was next door again!
I quickly ran around the house and into their back garden to fetch her. They were playing happily, but I thought her behind was a little wet - and some time had passed, since I last saw her, so I called the vet.
This time we decided to order a scan for her, the 21st of May we will know, if she is pregnant. If so, we must have it terminated. It saddens me a bit, but we never wanted puppies with her, and if so, it should be with another CKCS, not a dog of indefinte race. If she is NOT pregnant, we will have her spayed that day! Besides she is very young (from 30/9/2006)!
Once again I have found some garden stones and blocked the new hole and potential ones... ahhh - the joys of having a teenager! ;)