It's usually in a folder labeled "Operational" or "System Programs"
In the search box in marketing activities, search "lead score" or "lead scoring" or just "scoring" and the program should pop up.
If it is in a program that is.
Darrell Alfonso 's probably right, assuming your team follows logical naming conventions. But probably worth noting that there's no set location in Marketo where your organisation's lead scoring will be handled (beyond that it'll be in Marketing Activities, at least). If Darrell's tip hasn't helped you find it, then it's more an internal business question than it is a general Marketo question - find the person who built the program, or if all else fails, just start hunting through folders.
As Darrell and Grace pointed out, your Lead Scoring model should be centralized, although it is still possible to adjust lead scores from any smart campaign (not best or recommended practice).
If you want to find where your Lead Score is adjusted you can do this from Field Management in Admin (should be /#FM0A1 when logged in). Once you choose the field Person Score (previously Lead Score) you can see which campaigns it is used in. In the list, you will find the active campaigns using lead score with the lit up lightbulbs, looking like this:
Hope this helps!