[JohnFen]

No, there's never a sign. Like with all tipping, you're supposed to just "know" somehow.

But (at least then -- I've noticed that things are a bit different now) there is usually a little card telling you the name of your housekeeper. That's where you're supposed to leave the tip (if you're leaving one). Money on the card was deemed a tip, money left anywhere else was deemed forgotten property and turned in (if the housekeeper was being honest).

Where I worked (at a Hilton), we had to clean 17 rooms per day. If every room left a $1 tip -- which never actually happened -- that would come close to doubling my wages for the day. It made a significant difference.

[symic]

I see signs now asking for a tip in hotel rooms. Lots of tipping relies on signs. Like at the register stand where the iPad is swiveled around asking for the user to input a tip percentage.

By signs I mean things like you mentioned nudging one to tip. It’s not an overt sign stating it is expected but it is a sign letting you know that it is expected.