Expanding on that idea a little. Store the proof of operation on the clients own server.

Let them enter a url to a script on their own server, that you will POST to, whenever you check their server. The user can log POST's to a text file, DB, etc., so they know that you are actually monitoring them.

When they see that you haven't POSTed to that file for a while, they know you're out of business, and they are unprotected.

On second thought, make it easy on yourself, and use GET. It's not like there's anything confidential about it.

I've been thinking of implementing webhooks as an additional form of notification, but not as a a presence mechanism. I like the idea, but given the simple nature of the service I'd have to give it some thought before implementing something like this.

I'll give the "last checked" idea some thought. I'd need to store some additional data as I'm currently storing only abnormal behavior.

Regarding the weekly/monthly update: this is something I'm averse to. Other services do this and it kind of bugs me each time I get these.

I'm with you. I think the simplest solution is best here. I do expect you'll run into scenarios where last-checked-time will be data you'll want anyway...

Another option might be a weekly uptime report. That way I still know you're in business and I get some useful information.

Sounds like a premium feature to me ;-)