I’m not an expert on modern alarm systems but it seems that it is very common and fairly inexpensive to have cellular data backup. Not every system has it, but many do. In that case cutting the main connection will likely result in someone appearing on site fairly quickly.

Many cameras also have some form of local buffering. So even if you are gone before someone does show up you still may find yourself recorded.

But at the end of the day just put a bag over your head and you can be gone by the time anyone shows up without leaving a meaningful trace. Other than the very top-end system security systems just keep the honest people honest.