That'd seem like an obvious one, only... we don't have any other regular cat (or dog) visitors to our garden. Our Nigel is also massively weird about / hostile to other cats. She'll sit in front of them, making noises like a rat in a thumbscrew, until they get weirded out and leave (most of the other neighbourhood cats seem friendly but bemused).
She's like this in all weather, and has been like this for just under a year. Regardless of the weather. And I'd kinda expect snow, wind and rain to wash away much of the remaining scent, if there'd been some there to start with.
She ALSO sometimes gets up on her hindfeet, and sniffs higher leaves. And flehms them, too. And she'll sniff around every side. Wouldn't that require a prolific quantity of piss, and a cat with an inordinately high aim?
FINALLY, she clearly finds great security in the bush. As shown in the pics, she's recently started hiding in there. We'll go out into the garden and start calling for her, and suddenly this miaouing will start coming from the bush. And if it was coated in another cat's piss, I'm not convinced that'd happen. Would it?
She's never pissed there either, btw. Or, at least, I've never seen her pissing there; my ground-floor 'office' window looks straight onto that bush (which is about 3m away); and she's only recently started climbing out of view. We've both seen her pissing prolifically in our vegetable patch and the neighbour's rhubarb patch, though.
At that angle and lighting, it could be anything from privet to some kind of maple 
It's backlit atm, and I can't get any other angle without climbing into the neighbour's garden. I'm in a dressing gown atm, so that'd be looking for a quick call to the police.
