Hi Jethro and WELCOME!
A good short-wave antenna can be as stealthy as any other.
You have 2 excellent options that both work well; the end fed longwire and the loop.
It sounds like you don't have a balcony, so the outdoor stealth loop seems to be out, but you can string up a loop (can be circular, diamond, delta, or square shaped) indoors on the area closest to the outdoors and it will work pretty well. There might be plenty of metal in the make-up of the building which might actually help you in this case. If the window is metal, one simple sheet screw might help use that metal to your advantage as well. A loop "hears" at frequencies far lower than it's fundamental resonalt frequency. If you can make it 35' or so in total loop lenth, that's good enough to hear stations pretty well from 10m to 40m and then down to 49m SW with ease. You can use a cheap spool of radio shack speaker wire and it will do just fine.
The old adage of get as much wire as high and out in the open applies to all. With a 3rd floor apt. you already will have some height advantage. Can you get any wire at all outside? If you can, the speaker wire, when spit makes an excellent stealth antenna. You'd almost have to be right up on it to see it. Anything outdoors will work better than indoors for an antenna. Can you open the window? Are there any nearby trees to the window? Do you have access to the roof?
If so, any stealthy wire you can get to the roof or with a 2oz fishing weight--get into a nearby tree will help immensely. Just run the other side wire of the split speaker wire around the baseboards inside to act as a counterpoise.
Thinner wire will always be hard to see and stealthy.
I knew one SWL in a 3rd floor apt who ran speaker wire out the window and was close enough to an aluminium rain gutter downspout and just slid the wire beind the downspout almost to the ground and then drilled a hole big enough for a sheet screw and attached the wire to the screw under a small washer. He said it worked great because all the rain gutters were tied in together with all 4 downspouts...over 600' total of aluminium!
Good Luck and let us all know what you did and how it worked!
ex-KA0NIU, ex-KA9SOX, now K5TEN (117 countries confirmed--Submitting for DXCC soon! WOOT!)
SWL: WDX9KJX And the "WDX9KJX Short Wave Monitoring Service" from 1973 to 1986
1st SW QSL: "Happy Station" Radio Nederland Wereldoemroep, Holland, 1974
2nd SW QSL: "The Voice of Nigeria" Lagos, 1974
3rd SW QSL: "Radio Moscow" USSRhttp://m3jzt.proboards82.com/index.cgi