Portable AC leaking water....how to locate problem?

Have an older portable AC — maybe 10 yrs old. Has been sitting in the garage unused for a couple of years. Gave it to my son to use and after a couple hours of running he reported there was a large puddle of water under it but the collection bin for the water (that needs to be manually emptied and sounds an alarm when it needs emptying ) was bone dry.

I tried 2 tests. First one i ran in the garage for a couple of hours. No leak…but also no water in the collection container. When preparing to bring it inside for test 2, I tipped it and water ran out, confirming that water was going somewhere….just not to the collection bin. I brought it inside and put a pan under the unit to catch any leaking water and ran it for a couple of hours. When i checked I found a puddle under the unit, no water in the tray I put under it, and the unit’s collection bucket was bone dry. When i was taking the unit back out to the garage and tipped it to get it down some stairs, the collected water came out.

Any idea how i can locate the leak? Do these things operate if I pull the back off and try to run it for a couple hours? It’s as if there is a hose inside the unit that is supposed to direct the condensate to the collection container but it has dislodged. Is that a possibility? thanks.

More info + pics:

See info on top pic. It makes sense if the secondary drain hose has a plug with a small hole in it, water will come out. I tested this.But what is puzzling is this thing has never been opened up. This is all original equipment. And it has never leaked before. The other odd thing is when i poured water into the condensate tray, it drained to the secondary hose first. I covered the hole in the secondary hose plug, and water eventually started flowing, as expected, into the condensate container. I figure the secondary hose just filled up, backed into the condensate reservoir, and found the drain hole into the condensate container.

So do I just plug the hole in the secondary hose’s plug? At least that seems like a place to start I suppose.

It is just so odd as there are no issues or rotting hoses, yet this leaking has never occurred before. And why is there a hole in the plug on the secondary container if it isn’t supposed to be there?

thanks.