It also really, really depends on what kind of "guarded area" situation we're discussing here.

If it's a noble's estate, or a museum, or similar, the guards are probably here every day, for years. It's their day-to-day job. They probably have dealt with one or two intruders in that time, but not very often. This isn't a situation where they are likely to raise the alarm and put the whole area on high alert because they heard a weird noise. In the case of sudden silence, they'd probably pause to investigate rather than immediately running to the alarm station or whatever.

On the other hand, in a much "tenser" situation (guarding traveling VIPs, vital military secrets, a castle vs. impending invasion, etc) the guards WILL have a hair trigger. They're not afraid to put the base on red alert for a false alarm. They'll also likely have a much better structure for reporting alerts, with guards operating in teams. One or two guards will investigate, another will sound the alarm, etc.

In addition, Zeb has the right idea about the quality of the guards. Royal Special Forces will react differently from local rent-a-watchmen.

Of course drawing these distinctions can get tricky in-game, because there's a fine line between "your silence spell makes you unstoppable" and "The guards are too smart. Your silence spell is completely useless." Either extreme can get a little frustrating.

Hmm, here's a thought: silence doesn't work around corners, right? Would it be possible to build a kind of "Bull's-eye Lantern of Silence?" Basically a small lead box with a controllable opening, through which the Silence effect would emanate? I know it works for the Light spell.