Sorry for the late reply.
Yes, the returns have a pull up resistor so they would normally be sitting at a high as long as all the switches are open OR the switch matrix connectors are unplugged.
If you found one low in this condition, it would indicate a damaged input chip. And more than likely, would show every switch on that return being reported as closed.
As to the strobes, they are continuously pulsing as only one strobe is "ON" at any point in time. The CPU turns one strobe on (low), and then reads ALL the returns to see if there are any closures (inputs low). If a switch is closed, it allows the low strobe to drive the Return input low (instead of its normal high state). It then turns that strobe off, and turns the next one on and re-reads all the returns again. It repeats this until all the strobes are scanned and then starts over. This is how the manufacturers could use up to 8 ins and 8 outs (total of 16 I/O pins) to read up to 64 switches.
So, in normal circumstances, the strobes should all be pulsing and not be either high or low. The state or appearance of the returns are all going to depend on if or how many switches are closed out in the cabinet. The more of the switches that are closed on that return line, the more "Low" your Logic probe is going to show. But it should be either always high, or plusing, never stuck low. This is, however not true of the Outhole or slam switch input as they are special switch inputs wired directly to a ground source.
Remember, is some designs, the dipswitches are also placed on the switch matrix or at least read upon power up via their own select strobes or returns while sharing some lines of the switch matrix. This is NOT, however the case on a N-iwumpf board but is the case for an original Gottlieb System 1 MPU board.