PCB power planes and routing question
With an unpowered populated board, you may get all sorts of odd resistance readings between pins. Each IC will have protection diodes which could conduct at resistance-reading voltages, and may have internal resistors connected to pins which conduct at any voltage. Try altering the measurement conditions to distinguish these. The first simple test is to reverse the meter probes. A 515ohm resistor will behave according to Ohm's law if you add another resistor in parallel and then one in series, whereas diodes tend not to.

The implication being, as Chris Knudsen has pointed out explicitly, do it without components on the board if possible.