Merry and Pippen's escape from Grishnakh

I am half way through reading LOTR for the second time. For the second time now I have become stumped at the same place. Merry and Pippen are taken by Grishnakh while Ugluk was busy during the raid by the Riders of Rohan, because he knows about the ring and wants is for himself. When he is far enough away he starts fingering the hobbits in search of the ring. Pippen guesses that this is what he is doing and tries to work the situation to his advantage.
"'I don't think you will find it that way' he whispered. It isn't easy to find'
'Find it?' asked Grishnakh: his fingers stopped crawling and gripped Pippen's shoulder. 'Find what. What are you talking about little one?'
For a moment Pippen was silent then in the darkness he made a noise in his throat:gollum,gollum. 'Nothing, my precious,' he added.
The hobbits felt Grishnakh's fingers twitch. 'O ho! Very, Ve-ry dangerous, my little ones.'"
The first time I read that (it seems stupid now, but I thought maybe Gollum was watching and was going to kill Grishnakh to save "his precious" and the hobbits at the same time.)
Anyway, the thing that stumps me was the mention of Gollum (or atleast Pippen making the gollum sound in his throat). Did Pippen really think that Gollum was there? Did he guess that Grishnakh was around when Gollum was tortured to find out where the ring was?
These are the two best reasons I can think of for this. Does anyone else have agree or have any other ideas?