and will peek the hotel staff interests.

It is better to wait somewhere else nearby, i.e. a gas station, a convenience store, or mini mall.

If that is not possible, then head directly to the hotel restaurant and have a drink, and when you receive the call to head to the room, pay for the drink, and head directly to the elevators.

Remember you do not want to attract attention to your self.

That's what I usually do in these situations.

Recently, I arrived 20 minutes early for a hotel incall (traffic was lighter than expected).  I reclined my seat in my car and waited about 15 minutes for her to call me back with the room number.  That got me thinking.  If a hobbyist has to wait a while for the call-up, is it better to wait in the car or in the hotel lobby?

Sitting in your car parked for more than a few minutes may get noticed.  If you are early, better to take a walk to a nearby store if possible.  Same applies if she is running late, which you will often have to deal with.  If you are going to meet her at a hotel, best to chill and have drink at the bar if there is a delay, or find stores nearby to grab a bite to eat.  Do not linger in the lobby unless it is crowded enough so that you will not draw attention.  And don't hang around outside, either.

reading.  As long as you act like a paying hotel guest and don't arouse suspicion, you should be okay.

I usually arrive early.  I would rather be 15 minutes or so early than late, especially since you never really know what sort of traffic mess you might get caught up in.  Usually I try to find someplace else to go near the hotel.  Often there is a mall or shopping center close by so those are good places to go.  If I'm running really early I can check out some stores while waiting.  A couple of times I wound up even buying a little gift for the provider while I was waiting.