Remote peers and mining question with playground network

Question

I am running a stagenet with 2 nodes for playing / learning Monero. First, the nodes should have been running isolated from the outside world. What I don't understand from the following logs:

1. What is this (95.115.74.43) IP? Is it a stagenet or Monero node IP?
2. Second, I am not mining, and thus I wonder how could a block be found (12:55:00.179)? Does it mean that somebody is mining on my playground node? Does a block found means there is a new coin being transferred to a wallet?
3. Is it a must to use stagenet?

2019-06-08 12:55:00.177 T dbg <<< global-IN: speed is A=  64.339 vs Max=8.38861e+06  so sleep: D=-9.65191 sec E=     621 (Enow=    1242) M=8.38861e+06 W=   9.652 R=8.09662e+07 Wgood      11 History: [621 0 0 0 0 0 0 0 0 0 ] m_last_sample_time= 44951.7
2019-06-08 12:55:00.177 D [xxx.xxx.xxx.xxx:8080 OUT] LEVIN_PACKET_RECEIVED. [len=588, flags2, r?=0, cmd = 1002, v=1
2019-06-08 12:55:00.178 D [xxx.xxx.xxx.xxx:8080 OUT] REMOTE PEERLIST: TIME_DELTA: 0, remote peerlist size=3
2019-06-08 12:55:00.178 D [xxx.xxx.xxx.xxx:8080 OUT] REMOTE PEERLIST: 88ebe20ef4fa7c68  **95.115.74.43**:xxxx   rpc port -  pruning seed 0  last_seen: never
69e17fad298bb67 xxx.xxx.xxx.xxx:xxxx    rpc port -  pruning seed 0  last_seen: never
f4068cf7ba6d1fcb    xxx.xxx.xxx.xxx:xxxx    rpc port -  pruning seed 0  last_seen: never

2019-06-08 12:55:00.178 T Blockchain::get_current_blockchain_height
2019-06-08 12:55:00.178 T BlockchainLMDB::height
2019-06-08 12:55:00.178 T BlockchainLMDB::block_rtxn_start
2019-06-08 12:55:00.178 T mdb_txn_safe: destructor
2019-06-08 12:55:00.179 T Blockchain::have_block
2019-06-08 12:55:00.179 T BlockchainLMDB::block_exists
2019-06-08 12:55:00.179 T BlockchainLMDB::block_rtxn_start
2019-06-08 12:55:00.179 T mdb_txn_safe: destructor
**2019-06-08 12:55:00.179   D block <95e3b029806384fe898d8efc53da37c2273369e114932fd21b6438e24fd8897b> found in main chain**
2019-06-08 12:55:00.179 T [xxx.xxx.xxx.xxx:xxxx OUT] [levin_protocol] <<-- finish_outer_call

Answer 1

the nodes should have been running isolated from the outside world

What you are describing is called a private network. See this answer which includes a link describing describing how to run a private testnet network. The key parameters to use are --add-exclusive-node ... and --hide-my-port when starting each of your nodes, as this ensures your nodes only connect to each other and do not get announced.

So from the information you provided in the question, it appears you are not actually running a private network. This is why #1, you have received a peer list from a remote node and #2, someone else on the network being used has mined a block, not on your node, but on the their node and relayed to the connected network.

Lastly, when running your own private network, #3, no it is not imperative to run as stagenet. It is certainly more common to run a private testnet.