I have found that with the Uno, Mega and RPi3B+, when I put an LED onto a unprogrammed pin, wonkies can happen; such as dimly lit LED's.
On an Uno A4 and A5, if I remember correctly, serves as I2C pins and one of those pins would be a clock, which would toggle the LED at about 1/2 brightness with a 50% duty cycle.
Those are my guesses.