You need to use the present, if the thing still exists. You could possibly use past if it doesn't exist any more. If 'the cake' is a specific cake, that has been eaten, then you could use past. If the cake still exists, or 'the cake' means 'a specific type of cake' (which of course always exists), then you can't use past. You are misusing 'found' here. 'Found' means something like 'are able to be read'. You can't say 'have been found' - that suggests they were lost. You could say 'were found' if, again, the box no longer exists.