Squirrel away
Definition 1 of 1
Pro Tip 1/3Secrecy Implied
The phrase suggests hiding something carefully, as squirrels hide nuts, not just saving. 
She's been squirrel away extra treats for a surprise party.
Pro Tip 2/3Long-term Focus
Often used for actions with a future goal in mind, like retirement or investment. 
They're squirreling away funds for a vacation next year.
Pro Tip 3/3Not Just Money
Can be used for collecting other things discreetly, not only cash. 
He’s been squirreling away rare comic books in his attic.