As a female who has had to deal with guys being odd and/or presumptive, DO NOT DO WHAT YOU'RE CONSIDERING. No one is interested in someone showing their interest via covert ops/subterfuge. You want to attract the ladies, not have them assume you're trying out for MI5 or the CIA.
1. If you're nervous, start small. Say hello when you see her. Work your way up to a proper conversation, but don't drag this on for ages. She'll think you're not interested in anything more than exchanging pleasantries and if you're inconsistent she may think you're a weirdo, at least I do when guys say hi one minute then ignore me the next time they see me.
2. ASSUME NOTHING. Again, this comes from my own personal experience. You don't know her, you don't know how she'll respond, you don't know what she's thinks of you. So don't assume that you do. When you do, you're the one that becomes the judgmental ass, particularly if you assume the worst of someone. I've never been flattered by someone saying, "I didn't say something sooner because I thought you'd think I was..." It's actually quite insulting.
3. Once you're comfortable, be direct and honest. Females are surprisingly responsive to both.
4. Subterfuge NEVER WORKS. Spying, sneaking, leaving messages, anything other than open communication can be construed a number of ways including: you're weird, you're a stalker, you're insecure, you're just screwing around for funsies/due to boredom. At the very least it results in mixed messages being sent, and again, will make her question your sanity as well as your motives and she is most likely not going to be amused. Especially if she's made to respond in kind, i.e. leaving messages. Not cool at all.
5. Failing to follow 1 and 3, in particular suggests to the ladies that you don't think she's worth the time or effort to be honest. Believe it or not, it comes across as half-assed, and insincere.
6. As I've said in other posts(you guys ask alot for a lot of relationship advice.

), don't drag this on forever, even if she is interested she will lose interest particularly if you're doing odd things, and there are other males out there. Don't assume she's going to patiently wait around for you to put on your big boy pants and make a proper move.
Finally, I'm not saying you won't be declined, but at least you'll know once and for all. And the next time you can be a bit more confident in your approach. And there will be a next time. So if this falls through, move on to the next.