I'm looking for a way to detect invisible units. BUT. I want to a detection square around 7x7 squares ( normal sized ) and I want it to last for only a few seconds.

Spawn//remove a detecting air unit. I suggest an un-upgraded observer.

I believe that preplaced Scanner Sweeps will never die, but they might not be movable. If they don't die and can be moved, then you can just move a Sweep to wherever you want detection.

Preplaced scanner sweeps disappear after running once at the start of the game, you also can't do any location related interactions to Scanner Sweeps, so that won't work.

Creating/removing the observer is your best choice, in my opinion, make sure you leave enough of a delay for it to detect.

I don't think observer's detection is exactly 7x7. If by detecting cloaked units you mean you want the player to see the cloaked unit, then it really depends on the map. If it's controlled enough you can have a 7x7 location on the spot you want, give the units in the location to a computer with whom you do have vision with. Then after the time interval you want, give it back to the original computer.
If you can't do that, then you're gonna have to stick with observers even if the range is not 7x7.

Well I just said around 7x7, but I'll just have to try that method.
LOL, I had a hyperlink in my signature spot that said Delete and i clicked it and it deleted my post... But it was supposed to be a button. ( useless fact of the day )

Sci vessels: 10
Overlord: 9 (11)
Yggsdrasill: 11
Observer: 9 (11)
Missile Turret: 7 (I think)
Spore Colony: 7 (I think)
Overmind/Cerebrate: 8 (I have no idea)
Photon Cannon: 7 (I think)
Overmind Cocoon: 10 (I have no idea)
Floor/Missile Traps: 6 (I think)
Flame Traps: 3 (I think)
Only the air units are really of any use, since the rest are pretty much unmoveable. If you don't have to move, disable a flame trap, which I think should still give detection.

Detection updates every 30 frames, and only 1 unit can be detected per frame, and the units revealed are random. You don't have to have vision to detect. If you want to, you could have an observer over you with no vision to yourself, and another unit (drone gives 7 vision) giving your unit/area vision. If you want a diameter 6 circle, use a spider mine.

Essentially, you'd be detecting 9 square radius, with a 7 or 3 square view radius. Anything you saw would come in visible, and could be turned off by turning on/off the observer.

How would I have an observer with no vision to myself? If it matters, the unit the observer is over must have its normal range of sight ( with the exceptions of observers sight radius ).

I have also played a map ( Lord of the Rings ) where they slowly took away vision of an area, even though there were no visible units to be seen. Maybe that's what I need?

EDIT: NOT with the exception ( I think ) I have just confused myself...

You have a player run the Ai script Turn OFF shared vision for (them self). That is how you turn off vision of a player for that player.

But wouldn't that turn it off for every unit of mine?

Yes it would. It would be best to just use an un-upgraded observer, like I said way earlier

Well if that's the only way, then maybe so. It's just that, in the game the player would randomly spawn an observer above their head and go, wtf is this all about? Then from there on they would know what to expect when an observer spawns above them... ( if they were smart -.- )