This is a set of functions to search entire DB by simple keyword. If you need to find something from any records from any collections that you don't remember names, just load this functions and `searchAll('any keyword')` in Mongo console.

mongodb_searchAll.js

/************************************************************************
*   This is a set of functions to search entire DB by simple keyword.   *
*                                                                       *
*   Developered by Won Dong(fkiller@gmail.com)                          *
*                                                                       *
*   * Usage: searchAll('any keyword')                                   *
*                                                                       *
*************************************************************************/

function createOR(fieldNames, keyword) {
    var query = [];
    fieldNames.forEach(function (item) {
        var temp = {};
        temp[item] = { $regex: '.*' + keyword + '.*' };
        query.push(temp);
    });
    if (query.length == 0) return false;
    return { $or: query };
}

function keys(collectionName) {
    mr = db.runCommand({
        'mapreduce': collectionName,
        'map': function () {
            for (var key in this) { emit(key, null); }
        },
        'reduce': function (key, stuff) { return null; },
        'out': 'my_collection' + '_keys'
    });
    return db[mr.result].distinct('_id');
}

function findany(collection, keyword) {
    var query = createOR(keys(collection.getName()));
    if (query) {
        return collection.findOne(query, keyword);
    } else {
        return false;
    }
}

function searchAll(keyword) {
    var all = db.getCollectionNames();
    var results = [];
    all.forEach(function (collectionName) {
        print(collectionName);
        if (db[collectionName]) results.push(findany(db[collectionName], keyword));
    });
    return results;
}