In the story "Little Lost Robot" from I, Robot, a modified Nestor (NS-2) hides himself in a cargo with 62 other physically identical robots.
I think one of the main reasons why the hiding robot was not easily identifiable was NS-2s did not have any serial numbers.
In the story general mentions:
‘But we had to have robots of a different nature. So just a few of the NS-2 model, the Nestors, that is, were prepared with a modified First Law. To keep it quiet, all NS-2s are manufactured without serial numbers; modified members are delivered here along with a group of normal robots; and, of course, all our kind are under the strictest impressionment never to tell of their modification to unauthorized personnel.’ He wore an embarrassed smile, ‘This has all worked out against us now.
It seems only a few NS-2 had modified first law but all of them were missing the serial number. I am not clear on how not having serial number was helpful to keep the operation quiet.