I'm retired so I don't have an office to hide things away from home.  I have my hobby phone & a few supplies (especially condoms, I always bring a couple)...  stashed in a nondescript nylon bag (I have other similar around the place) in an out of the way drawer in my workshop...  
Where do others with a SO keep their phone & etc?
skb

If you play golf, you might try your golf bag. Fishing, maybe a tackle box. A motor cycle, perhaps? In the saddle bag, maybe? One piece of advice. If you hide the bag in a drawer, then it looks like you are hiding something. Hide it in plain site, and then you won't look guilty, if you know what I mean. As for condoms, I would recommend not having them around at all. There are plenty of places that you can purchase them on the way to your appointment.
Hope this helps.

get a small storage space and keep it in there with other stuff, you could get a private mailbox and keep it in there, you could build a cubby in your garage or toolshed, you could put it in the spare tire well of your car, you could get a safety deposit box and keep it there.  

You could also just man-up and tell your SO to mind her own fucking business and you will fuck whomever you damnwell please....This approach is not suggested unless you are REALLY REALLY RICH, or HUNG LIKE A SEX SHOW MULE!!!

LOL

remember to turn it off before you stash it.  Also, remember to wipe the memory each time and put in a completely random password so that you can say that you found it in the gym parking lot and that you were going to turn it into lost and found when you go back.

I used to hide my hobby phone at work, but recently ascribed to a theory proposed by another hobbyist-  in sight and normal.  Told my wife it's my pager and keep it on a charger by bedside-  but it is a pda that is password protectd- even if she tried, she wouldn't be able to get in to it.

Rest of supplies- in a box hidden deep in my closet.

I have lots of bags around the house (I work out a lot).  One of my bags is a backpack for my computer that I take when I travel.  I keep my phone buried in the bag, under a bunch of cables and chargers and other junk.  I doubt that it will ever be discovered.  I don't keep condoms anywhere in the house.  I had a vasectomy, so explaining the presence of condoms would be impossible, unless I blamed it on my daughters (which I'm not willing to do).

One more trick to support the "I just found this phone and was going to turn it in but I forgot" story is to write someone else's name, or the name of a business, on the phone.  If it's someone's name, obviously use a fairly common name.  If it's a business, either pick some large business (e.g., "Avis Rental Car"), or make up a fictitious business (e.g., "Greene Plumbing").

I use toys with providers and a briefcase does not look suspicious in a hotel.

to stash condoms?  Every provider I have visited has had them available.  If you have a SO, that's just asking for trouble to keep them around, even if well hidden ...

I keep an "all purpose" box in my trunk covered by miscellaneous items like a hat for jogging while away from home, towel to wipe the car windows, extra travel coffee mug, etc.

This approach is also suggested if you no longer want a SO. lol

I know a guy who keeps his hobby supplies in an emergency kit in the trunk of his car. Since there are real emergency supplies in the kit, the prepaid cell and charger don't raise any suspicions.

Every provider I've seen has condoms (one Fav asked me to pick them up which was inconvenient...  I'm known locally and did not know her neighorhood...  the rest was all highway).  
I've seen the one pop scam... a lady claims to be out of condoms so can't do more...
My last date, I let the lady use her condom & now I remember why I was buying the premium thin ones...  made a big difference for me.  
You are right that the cost is low enough to disguard extras.  Maybe I need to generate a location to stop enroute to buy them...
skb

As to condoms, I just store them on my penis. The wife NEVER thinks to look there...