I am a pretty heavy business traveler and stay in moderate priced motels to 4 star hotels and I get follow up calls from the front desk from many of them. Don't know why your client got spooked, maybe he was a newbie. Next time you check in, you might tell the front desk no need to call the room.

mid to upper scale hotels; especially if you are a member of their frequent guest program.  

If I'm traveling on business and are having company in my room and don't want to be disturbed, I pull the phone cord out of the back of the phones.

Take the phone off the hook.

That's how I cured it.

Yes, it happens in many hotels.  You can avoid this by calling the hotel operator when you get to your room and telling him/her that you do not want to be disturbed.

Sounds like your guest was a bit paranoid.

then he would know this is SOP at the nicer hotels.  Where does he usually stay, Super 8?

I've also had them while traveling with my family.  I didn't thik anything of it at the time but can see how it would be disturbing.  There's only 1 phone in most rooms...  I agree unplug it.  
skb

Tom Bodet(sp?) must be leaving a light on for that guy if he didn't know that a courtesy call from the front desk is in fact quite common.

Too bad that he got spooked.  I get one everytime I check in too....definitely take the phone off the hook or just tell them not to "courtesy call" you.
xoxo

yes many hotels do this. Just tell them all is fine end of story. Hampton Hilton and Sheraton are famous for that.