If you're getting your cock gobbled off in an inappropriate place and a guy walks in on you the thumbs-up is your best option. Sure, you could act like a goober and fumble for cover (thereby ending the blowjob and telling the girl you like sucking cocks as much as she does). When you thumbs-up the guy you're speaking to him in an ancient code of men. A gesture as simple as the thumbs-up is the silent method of saying to the guy "Hello my fellow man. I didn't plan for this but this chick just started putting tonsils to my knob and I know you, as a man that I assume appreciates vagina, can respect this situation and be happy for me. Thank you for quietly leaving; I'm sure this has happened to you many times so I know you understand."
If a woman walks in on you and you thumbs-up her you're saying something entirely different; "Hey, you look like a really cool woman. Much classier and cooler than this skeezer I'm fucking in the mouth. Even though you're a woman I respect you as if you were a man and assume you can understand how cool this is." This is risky; if she's a feminazi she'll call you a disgusting pig or try to ask the girl why she's blowing such a loser. At that point your only option is to feign outrage and be like "Hey now! Me and my lady are enjoying an intimate moment and it's none of your business!" and hope the feminazi just gets frustrated enough to leave. Once she leaves you say to your girl "The nerve of her, she must be a lesbian!" as you gently push her head back down.