Notify when office is empty

We are small studio that rents out our basement to another company who have their own entrance. The alarm is on our floor, so we want a simple way to notify the other company when the office is empty (so they can turn off the alarm).

Ideally when they get near the office a sms or notification will be sent if it is empty.

I have some vague ideas of how to approach this.

1) Use ifttt location channel to send sms (does not work well for multiple people)

2) Somehow check when each phones connect to office wifi (not sure how to do this)

3) Create a web app that uses websockets to check if phone is present (really would need to run in background)

Anyone else got any other suggestions ?