Modified date on product items should uodate when the on-hand quantity changes
When you change something simple on a product item (say the description) then a request for modified since returns this item.
But if the stock in-hand figure changes (because you've sent an invoice) then the modified date doesn't change and this item isn't returned in the atli query.
So, in effect there is no easy to tell (via API using modified dates) when the stock on- hand has changed. You would have to load all items every ten minutes to know what the current in-hand stock is... And then you would get rate limiting issues.
Kyran O'Neill commented
This is also affected when you do manual adjustments to the stock