The phrase "constant undercurrent" refers to a continuous and hidden feeling, situation, or influence that is always present but not easily noticed. Full definition
The theme of natural exploration and conservation is a strong and constant undercurrent of Amok Island's artistic practice. (thisiscolossal.com)
Andrew Mitchell's demise was a 2012 story, true - but the steady unravelling of the case against him has been a constant undercurrent of 2013. (politics.co.uk)
There is a constant undercurrent of arguing, of jockeying for position and power, of battling over personal space, of loyalty and betrayal. (scientificamerican.com)