No. If the hotel thinks the room is vacant, you will get checked out. A good hotel might contact you to inform you that you left your backpack behind.

Remember, guests doing mattress runs are outliers. Guests leaving without formally checking out prior to the actual end of their reservation is a common occurrence. It happened all the time when I worked in hotels...particularly on Wednesdays and Thursdays in our business-oriented hotel.

One of my former hotel jobs was Income Auditor. In that position, I handled the credit card chargebacks. In cases when guests left early without checking out, we always lost if that person could prove they were in another city. It didn't matter that we were right. Every credit card company would rule against us. We didn't even bother fighting them.

In my opinion, that's the reason why hotels are vigilant about checking out rooms that appear to be vacant, even if the reservation is longer. Chances are strong they will lose any chargeback and they will lose the opportunity to sell the room to someone actually present. As such, I wouldn't expect to check into a hotel for a multi-night reservation, disappear, and assume you'll get credit for the entire stay.