Cargo Hold Trigger Trouble

Heya, I have a trigger question:

I want to make it so when a unit enters an orc burrow, an integer variable increases by 1. when they leave, i want it to decrease by one. i also want a periodic event to test how many units are in burrows (but thats just really testing the integer, if the other 2 work). So that's 3 triggers.

The first one, no problem. the third one, no sweat.

The second one - where a unit leaving a burrow decreases the integer, is giving me serious trouble.
Though there is a 'unit is loaded into a transport' unit event, there is no 'unit leaves a transport' unit event. i could shortcut these triggers and just test how many units are in burrows each time i want to, but theres no way to check how many units are in a burrow, much less all of your burrows.

any advice?

I am not sure but i believe that can use the "a unit is issued an order with no target" event, using a condition that check if the order is unload.

Actuall its "unit is issued an order targetting an object" for the "unload" order.