Oh yeah. Every single pair of shimanos I've had (5 sets) over the last 5 years eventually leaks around the pistons. They work great for about a year and then they start seeping. Riding them every day or every few days kind of keeps it at bay but if any of them sit for longer than about a week, they get fluid on the pads. Every single fucking pair. Let it go long enough without a bleed once they start doing that and then you also have air in the system that eventually gives you the wonderful engagement free lever to the bar effect. It's awesome. They've warrantied every pair I've sent in but I'm tired of it.... were you actually doing that?
It's not uncommon. Not at all.