By the time I get home, someone will probably be cleaning the garage.
Literal
I [subject-が] house [at-に] arrive time, someone [subject-が] garage [object-を] cleaning-is probably.
着くころ ('by the time [I] arrive') uses ころ ('around the time of') — less precise than 時, indicating 'sometime around when.' 誰か ('someone') as indefinite subject. だろう adds modal prediction. The ~ている future progressive is used for 'will be doing' rather than 'will do.'