How does your arsehole know you're home?

Imagine this scenario.

You're at work, you've just finished. Home's only around the corner. You need to pee kinda. You need to shit. Like neeeeeeed to shit! You can hold it. "I'd rather turd in my own bowl, much more comfortable." Those are your thoughts. Things are going well thus far, although you need to pee more. In the privacy of your car you've squeezed out a couple of farts that you were too nervous to do in front of other people you know, just in case. When you realise you haven't crapped in your pants and you get that gas-that's-gone-thru-shit-before-exiting-my-anus smell, you're kinda disappointed you didn't give the boys at work the last goodbye. You still need to pee. Busting for that crap though, but you're ok. Now you're home, you're in the driveway, you're sitting in the drivers seat bobbing up and down chanting "hang on hang on" to yourself while you get the front door key ready before you get out so you're in quicker. Do you know why you do this? You've held it fine. Then you get to the front door, and you will, repeat you will, for some reason fumble with your keys, and take about 10 tries to unlock a door that's normally easy. Now you're convinced you've just shit yourself. You had to go bad, but not that bad. Until now. When you're right at your front door. Your arsehole knows this and opens slightly. It knows you're home.

How the fuck does it know that?