It could very easily be a surprise still.
you can find people's registries without asking them--it's recommended all the time as a strategy for finding out. Just google their name and the word "registry.
Or, check quickly through the 4 big stores people usually register at.
(The Mother-to-be probably registered because of a family shower.)
Or, the coworker could have said, "are you having a family shower? where did you register?" without giving anything away. Or snooped through her desk. Or Facebook messaged one of her "friends" to ask.
So don't assume it's not a secret anymore, and don't tell her.
It's completely fine to give her something that was not on her registry. A registry is simply intended to be a list of ideas for the people who need one. Or guidance for people who are buying something that really needs to match (kitchen towels, china, etc.)
You didn't need an idea, and sippy cups, etc., don't need to coordinate with anything.
Have fun at the shower!