Hi Coyote - a few thoughts (hope the old hands don't mind my repeating myself on this).
It depends on what you are going after, how much you wish to invest in equipment and how time you can spend and how much risk you find acceptable (usually a combination of the likelihood of discovery and the cost of discovery).
You can go farther afield making neighbor recordings - having a cardioid or supercardioid microphone (shotgun mic's, parabolic mic's) can allow you to record neighbors at a distance. Temperate weather, open windows and an out of the way location from which to record all contribute to getting a good recording. Getting to know your subjects/targets helps as well... I've a neighbor that spent four weeks out of town on a construction project this summer - the couple are in the mid 20's - no kids... I imagine it was an energetic homecoming that evening. Unfortunately, thunderstorms and monsoon rains preventing an attempt at recording.
You have more targets in an apartment complex but need to be careful about creating suspicions walking around the parking lot with a 24" boom microphone
I've had one sucess with a neighbor recording. I've tried several other times but never came home with anything usable.
Motel recordings - had an experience several years ago with a loud couple next door. Beat the heck out of the movie on Skinamax... I've had some success recording couples from the hallway or corridor outside their room.
I like the target density - a decent sized hotel, on a weekend, might have 50% plus occupancy rate (higher in some cases). That might give you 125 rooms to start, discounting families, "party" rooms, etc you still have a decent number of room where people are having sex.
Timing is the key - knowing a couple are in the room is one thing, catching them going at it (with out getting caught yourself) it a wee bit tougher. "Do Not Disturb Signs" are pretty nice... you know the room is probably occupied and that the occupants are not as likely to come bursting out of the room.
I use a small amplifier (sonic ear, bionic ear, etc) to determine if a room is occupied and if so - if there is any activity of interest occuring... you can bend down and fix your shoelace, place the amplifier at the base of the door and get a clearer idea of what going on the other side. There have been several times when none of that was required - one gal I could hear half way down the hall.
Recording - I use a digital voice recorder and a small omni-directional or cardioid microphone that will fit under the door. Usally I'll adjust this so that it isn't obtrusive or obvious to anyone in the room. I then cover the microphone and recorder with a newspaper or magazine. I can then loiter less conspicously or attempt to find other recording subjects.
Pro's: I have gotten as many as three or four recording in an extended evening - depending on the hotel and it's customers (10 pm local time to about 1 or 2 am work for me) your best recording times will vary.
Variety - you get a wide mix of folks on the other side of the door... I've two recordings of threesomes (mmf and ffm) that I've posted. I've two recordings of two couples in the room having sex (not yet posted). I found one master/slave couple.
Risk - I've not had anyone affiliated with the hotel challenge me or question my actitities -- I'm not hanging around the hallways in a trenchcoat either. Placing the recording rig and not directly monitoring the couple (ie - dropping to the next floor to try and catch another couple), runs the risk of having your equipment discovered and taken (especially by the subjects
... Yeppie or someone else on the board had pointed this out to me earlier this year and unfortunately it did happen... I only had $ 50.00 invested in the equipment so I don't feel so bad about that - losing two great recording I'd made that evening still bums me out.
Cons - Hotel hallways can echo and amplify sound of other folks down the hall. Radios and Televisions - some couples just don't want to share their joy... they'll crank up the background noise. Spending some time with your sound editing software can salvage some recordings.
I'm sure other folks have additional insights, techniques and tips.
Good luck in your pursuits!
reg