The observed PIN

Alright, detective, one of our colleagues successfully observed our target person, 
Robby the robber. We followed him to a secret warehouse, where we assume to find 
all the stolen stuff. The door to this warehouse is secured by an electronic 
combination lock. Unfortunately our spy isn't sure about the PIN he saw, when 
Robby entered it.

The keypad has the following layout:

┌───┬───┬───┐
│ 1 │ 2 │ 3 │
├───┼───┼───┤
│ 4 │ 5 │ 6 │
├───┼───┼───┤
│ 7 │ 8 │ 9 │
└───┼───┼───┘
    │ 0 │
    └───┘
He noted the PIN 1357, but he also said, it is possible that each of the digits 
he saw could actually be another adjacent digit (horizontally or vertically, but 
not diagonally). E.g. instead of the 1 it could also be the 2 or 4. And instead 
of the 5 it could also be the 2, 4, 6 or 8.

He also mentioned, he knows this kind of locks. You can enter an unlimited amount 
of wrong PINs, they never finally lock the system or sound the alarm. That's why 
we can try out all possible (*) variations.

* possible in sense of: the observed PIN itself and all variations considering the 
adjacent digits

Can you help us to find all those variations? It would be nice to have a function, 
that returns an array of all variations for an observed PIN with a length of 1 to 8 
digits. We could name the function getPINs (get_pins in python). But please note that 
all PINs, the observed one and also the results, must be strings, because of 
potentially leading '0's. We already prepared some test cases for you.

Detective, we count on you!

The observed PIN - getPINs.js

// Requires >= ES6 (Set object)
function getPINs(observed) {

	var combos = [];
	var neighbors = {
		"0": ["8"],
		"1": ["2", "4"],
		"2": ["1", "3", "5"],
		"3": ["2", "6"],
		"4": ["1", "5", "7"],
		"5": ["2", "4", "6", "8"],
		"6": ["3", "5", "9"],
		"7": ["4", "8"],
		"8": ["5", "7", "9", "0"],
		"9": ["6", "8"]
	};
	var strDigits = observed.toString().split("");
  
	getCombos(strDigits, 0, "");
	return combos;


	// Depth first combinatorial traversal
	function getCombos(digits, idx, curCombo) {

		// Get possible candidates
		var curDigit = digits[idx];
		var candidates = new Set(neighbors[curDigit]);
		candidates.add(curDigit);

		//console.log(digits, idx, curCombo, candidates); // Pretty cool
		candidates.forEach(idx == digits.length - 1 ? reachedEnd : goDeeper);

		// (Avoiding anon funcs)
		function reachedEnd(candidate) { combos.push(curCombo + candidate); }
		function goDeeper(candidate) {
			getCombos(digits, idx + 1, curCombo + candidate)
		}
	}
}

/* Output of one test (with console.log lines)
[ '3', '6', '9' ] 0 '' Set { '2', '6', '3' }
[ '3', '6', '9' ] 1 '2' Set { '3', '5', '9', '6' }
[ '3', '6', '9' ] 2 '23' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '25' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '29' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '26' Set { '6', '8', '9' }
[ '3', '6', '9' ] 1 '6' Set { '3', '5', '9', '6' }
[ '3', '6', '9' ] 2 '63' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '65' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '69' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '66' Set { '6', '8', '9' }
[ '3', '6', '9' ] 1 '3' Set { '3', '5', '9', '6' }
[ '3', '6', '9' ] 2 '33' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '35' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '39' Set { '6', '8', '9' }
[ '3', '6', '9' ] 2 '36' Set { '6', '8', '9' }
✔ Test Passed: Value == '[\'236\', \'238\', \'239\', \'256\', \'258\', \'259\', \'266\', \'268\', \'269\', \'296\', \'298\', \'299\', \'336\', \'338\', \'339\', \'356\', \'358\', \'359\', \'366\', \'368\', \'369\', \'396\', \'398\', \'399\', \'636\', \'638\', \'639\', \'656\', \'658\', \'659\', \'666\', \'668\', \'669\', \'696\', \'698\', \'699\']'
*/

JSfiddle: https://jsfiddle.net/ryunp/8gah9zft/

Thanks! Here is a possible solution, using C+.