A recurring but elusive observation, felt over a long period of time

I'm looking for a word or phrase I think I've heard or seen, meaning something like, "a curious thing," but lighter, less forward. Like a passive observation one makes, of a handful of instances over a decade, but only lately worth acknowledging a pattern.

It's always been there, a ______, but ...

Solution 1:

How about

a part of the furniture

Collins

If you describe someone or something as part of the furniture, you are suggesting that they have been somewhere such as their place of work for such a long time that it is hard to imagine that place without them.

which they say is [humorous, informal].

I would use as:

That funny looking statue over there, has it been there long or is it part of the furniture?

Something that has always been there, but you just never noticed it before.

Solution 2:

"They've always been there, those fleeting moments of synchronicity/pause-worthy coincidence/subtle moments of perfect/uncanny coincidence, but ..."

https://en.wikipedia.org/wiki/Synchronicity

Synchronicity is a concept, first introduced by analytical psychologist Carl Jung, which holds that events are "meaningful coincidences" if they occur with no causal relationship yet seem to be meaningfully related.

uncanny

adjective having or seeming to have a supernatural or inexplicable basis; beyond the ordinary or normal; extraordinary: uncanny accuracy; an uncanny knack of foreseeing trouble. mysterious; arousing superstitious fear or dread; uncomfortably strange: Uncanny sounds filled the house.