Humans have been roughly at their current biological level of intelligence for at least thousands of years. But our ability to detect aliens has increased dramatically. Similar, a slightly more intelligence alien species that's lagging us by a few million years would have a hard time detecting us anytime.

Or, conceivably dolphins and wales might be more intelligent than us. Or you could conceive of a highly intelligent squid like species on another world. Problem is, if their whole world is an ocean, they are going to have a harder time with metallurgy or electricity.

(It's probably not impossible to build up technology under water, but it's probably harder and requires more cleverness and time. And even if it doesn't, because there's an easy way that humans just don't see; there are probably some circumstances that make it much harder. Eg assume a world that's all ice or desert, so it can only sustain a million people instead of billions.)