Certain index plays...
Are well known to some pit personnel and may tip you off as a counter. The most notable are selectively taking insurance and hitting vs. standing on 16 vs 10, for example. Those plays, combined with bet variation, are the ones that draw the heat in my experience.
It is correct in negative counts to hit your twelve against a 4, 5, 6. This will make you look like a dummy, but it is actually the correct play. This is just an example of how the correct basic strategy deviation may make you appear unskilled when you are anything but.
The point is, unless the pit personnel and surveillance are familiar with these plays, then playing correctly is your best option. You won't give up any EV and there is inherent cover built in.
Other, more esoteric plays, like splitting 9's against an Ace are actually appropriate in positive counts. (The index numbers vary with the system) These are sure to confuse those around you as well as the pit.
Point: Just play right. Forget cover. It costs you money.