Welcome to the forum and I'm sorry for your loss
Too bad you came to us through this way.
Rats can grieve in a variety of ways. If a rat has been ill for a while, I notice my rats seem to grieve (at least visibly) less because they understand it's the end for said rat. If the deceased rat passed away very suddenly, I notice more visible grief. Just like people, rats have different levels of attachment to one another. When my rat, Grumpy, passed away, I noticed the my two rescues (which were introduced and then moved into the cage only a few months before), while sad, were not as seemingly sad as the younger rats, the ones Grumpy had taken a liking to. I have pictures of Grumpy going out of his way to cuddle up against the babies and to groom them way more than his brothers, Ni Ni and Lil Chu.
Of course, all of this does not take into consideration that rats may not openly mourn as others do, similarly to people.
When bringing the body back for my rats to say goodbye, I'm often in tears. I've seen rats sniff each other's faces (their nose) and their eyes, as if looking for signs of life. I've had a couple of them nip the body, as if trying to wake them up. My rats will check out the body and then my more social rats will come up to me and sniff my face. Of course, this may be anthropomorphizing, but it makes me feel a little better.
Some rats have been known to "help" their elderly or sick cagemates. I've seen videos of rats dragging sick rats into hides with them, I've seen younger rats go out of their way to groom older rats who are incapable of grooming certain spots, I've seen younger rats move out of the way of older rats for food. The list goes on. Tumble may have been trying to bring Inknut into a hide as to help her.
Tumble may be just trying to "move on" so to speak. In the wild, they can't sit in their hole and mourn the loss of a loved once like humans can. Rats still have to forage for food, get water, make sure there are no nearby predators, and do their best to survive, despite being sad.