You meant in your original post that you saw trades going off between the best bid and offer on the ISLD book itself. I thought you meant you saw ISLD trades going off between the National best bid/offer from the L2.
Yes I would agree that those are hidden orders, though its hard to actually see it as you are trying to look from the bid/offer to the last match, back and forth, trying to catch the best price displayed right before a new match is posted, which is difficult.
I guess I shouldn't be so surprised, but I played around with entering some bids and offers while I had the ISLD java viewer up and I could not believe how fast it works. Its been awhile since I looked at the ISLD book over the net. As soon as I hit enter, the ISLD book was updated with my bid or offer. I mean as soon as I put weight on the key on my keyboard, bam there was the order in the ISLD. From my house to PD's servers in Orlando, to ISLD servers in wherever NYC I suppose and then posted to the web/java book. All before I even fully push down on my keyboard. Technology rocks.