According to the manufacturer, you can leave it plugged in all the time.  Here is what their web site says:

9. How long can I leave the Battery Tender Plus battery charger connected to a battery?
In theory, you can leave the Battery Tender Plus battery charger connected to a battery forever. Like they say, “Just plug it in and forget about it!” Practically speaking, it is a good idea to check on the battery at least once a week. Strange things can happen. Sometimes a battery can have a weak cell that won’t show up until the worst possible time. Of course, that time is usually when the battery is connected to a charger. If something goes wrong, then you have to deal with the question of the chicken and the egg. Which came first? Did the battery fail because it was connected to the charger or did the charger fail because it was connected to the battery?

No matter how good a product is, anything can break. With a battery and a charger connected together, it’s a much better idea to be proactive and anticipate problems, however unlikely they may be. In more than 99.9% of cases, nothing will go wrong. That still leaves about 0.1% where something might. Learn to respect electricity. A little common sense can go a long way.

Call it what they like, but it is a lead acid battery, only the acid is soaked into a glas mat. And the last I knew, all AGM batteries do have a vent, only the vents are valve regulated, only venting under x amount of pressure, which is why they can be laid on their side w/ no leakage.

If it was me, the tender would stay on the battery all the time. Thats what I've done the last 10 yrs. It has been proven that if a battery stays full charged 100% of the time, they will not sulfate, and virtually last forever. As this is impossible when being used, they won't last forever, but they can last 10 yrs no problem,.....which I, and others have proved.